(1) Field of the Invention
The present invention generally relates to photo detecting semiconductor structures. More particularly, the present invention relates to photodiodes built using modern state-of-the-art complimentary metal oxide semi-conductor (CMOS) fabrication processes.
(2) Background Information
A key component of imaging systems is a photodiode detector--a device used to detect incident photons of visible light that originate from an object that has an image to be captured. The photodiode is exposed to incident light which travels through a transparent oxide isolation layer thereof and into a silicon region thereof that may include a P-N junction. When the P-N junction diode is reverse biased, a depletion region may be formed and electron-hole pairs are generated both inside and outside the depletion region in response to incident photons of visible light impinging onto the transparent oxide isolation layer. The photo generated electron-hole pairs are swept away by diffusion and drift mechanisms and collected in the depletion region, thereby inducing a photo current representing a portion of the image to which the photodiode was exposed.
A significant factor contributing to a sensitivity of a photodiode is its ability to capture as many incident photons as possible. The sensitivity of the photodiode is affected in part by dark current which is the amount of reverse-bias diode leakage current induced in the photodiode when the photodiode is in the dark, i.e., current that is not induced by light. Dark current is particularly generated at an interface between the transparent isolation layer and the depletion region. The dark current causes noise in the signal generated in response to the exposure of the photodiode to light. Excessive dark current may also cause the readout dynamic range to be reduced. Efforts are made to minimize the dark current thereby reducing the noise in the signal detected and preserving the readout dynamic range.
It is desirable to decrease the amount of dark current generation at the interface between a depletion layer and transparent isolation region of a photodiode.